Orbiting satellites are an important aspect of modern communication systems. Originally used for "single-bounce" communication, with a signal going up from one place on the surface of the earth and coming down in another, communication satellites are now being used to form complex networks in space, with each satellite in the network being able to communicate with many, but not all, of the other satellites. Optical inter-satellite links, with their high directionality, high energy efficiency, and tremendous information bandwidth, allow satellites to talk to one another, and to transmit a much larger amount of information. Optical Frequency Modulation (FM) links, a new concept, offer a way to transmit not only digital signals, but also analog signals, and to do this with a much higher signal quality than was heretofore possible. However, effective optical FM links have not yet been demonstrated primarily because of the lack of appropriate photonic components, such as FM optical sources, limiters, discriminators, and receivers.
The primary function of most of these systems is to transport analog information data from one point to another. It would be very beneficial to have an approach that provided for the transport of this data in its original format, without having to convert it first into a digitally encoded bit stream, and then reconverting upon reception. Conventional modulation schemes do not have sufficient Signal to Noise Ratio (SNR) to allow this, which is why the majority of this data is digitally encoded before transport. However, FM techniques offer a "processing gain" that can increase the SNR by 20 dB or more, thus allowing direct analog transmission of the data in many of these applications. This offers a considerable simplification in system hardware, an increase in system flexibility, and a reduction in cost.
Optical links using FM and Phase Modulation (PM) have been discussed in the article by R. F. Kalman, J. C. Fan, and L. G. Kazovsky, entitled "Dynamic Range of Coherent Analog Fiber-Optic Links", J. Of Lightwave Technology, Vol. 12, p. 1263 (1994). Their approach uses a conventional technique employing a local oscillator offset from the signal frequency by a difference equal to the Intermediate Frequency (IF), together with a limiter, a filter, and an envelope detector.
Referring to prior art FIG. 1, input signal 10 is combined with light 12 from a local oscillator (lo) laser at directional coupler 14. The combined signal is mixed at photodetector 16, amplified at IF amplifier 18, (optionally) filtered by filter 19, limited by limiter 20, split into a delayed and undelayed signal, with the two signals (delayed and undelayed) being mixed (multiplied together) in a final RF mixer 23. To get an FM processing gain, the system shown in FIG. 1 must have an IF that is much larger than the baseband modulation frequency. The square of the ratio of the IF frequency to the baseband bandwidth is the SNR improvement that one gets with the FM approach. Therefore, if 20 dB of noise suppression is desired, and if there is a baseband bandwidth of 20 GHz, an IF frequency of 200 GHz and a bandwidth for all the IF components of twice this, namely, 400 GHz, are needed. Amplifiers, limiters and envelope detectors that operate over a 400 GHz bandwidth simply do not now exist, so that such a system cannot be presently realized.
Therefore, there exists a need for an effective FM receiver to help further realize optical FM inter-satellite links. The present invention provides a unique solution for such need by providing an all-optical FM receiver that performs the same function in the optical domain that a conventional FM receiver performs in the Radio Frequency (RF) domain.